Sheet post-processing apparatus

ABSTRACT

A sheet post-processing apparatus comprises a pair of alignment plates for aligning the plurality of sheets conveyed from an image forming apparatus at the two lateral sides, the pair of alignment plates being movable by a predetermined distance in a direction perpendicular to the direction of conveyance of the sheets, and a stapler also movable along the leading edge of the aligned plurality of sheets. The stapler moves according to the alignment position of the sheets and staples the sheets at a plurality of positions corresponding to the alignment position. As the stapled bundles of sheets are sequentially discharged, they are stacked on a delivery tray in a zigzag manner.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2005-368278, filed on December 21, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a sheet post-processing apparatus for performing a post-processing operation on the sheets of paper delivered from an image forming apparatus such as a copying machine, a printer or a composite machine.

2. Description of the Related Art

Some image forming apparatuses that have been marketed in recent years are accompanied by a sheet post-processing apparatus for sorting the sheets carrying an image produced as a result of an image forming operation thereon and/or executing one or more than one post-processing processes such as stapling sheets. Such a post-processing apparatus is normally arranged downstream relative to the delivery section of the image forming apparatus main body.

A sheet post-processing apparatus for stapling sheets is typically adapted to align a plurality of sheets (a bundle of sheets) by means of an aligning means, staple them, and deliver them onto a delivery tray, where stapled bundles of sheets are laid one on the other.

In such a post-processing apparatus, a succeeding sheet is subjected to a post-processing process only after the completion of the post-processing process of the immediately preceding sheet. Additionally, the post-processing apparatus may or may not be provided with a stand-by tray arranged on the way down to the stapler for the purpose of holding the sheets of paper delivered from the image forming apparatus on a stand-by status.

Japanese Patent No. 2879969 describes a structure to be used for a stapling process. The structure of the above-cited Patent Document includes a binding means for binding the sheets of recording paper delivered from an image forming apparatus, a moving means for moving the binding means along a plurality of binding positions for binding sheets of recording paper and a position control means for controlling the position of the moving means so as to align the final binding position of the current sheets of recording paper with the starting binding position of the next sheets of recording paper.

While the structure of the above-cited Patent Document can reduce the moving distance and the moving time of the binding means, the number of the stapled sheets to be laid at a delivery tray needs to be limited because the bundles of stapled sheets of recording paper are delivery at the same binding positions and, when bundles of stapled sheets of recording paper are delivered and laid one on the other at the delivery tray, the bundles are heaped up higher at the part of the binding positions than at the remaining part of the sheets. Additionally, the above-described structure is accompanied by a drawback that a succeeding bundle of sheets that is stapled and delivered to the delivery tray forces out one or more than one uppermost bundle of sheets already delivered to the delivery tray.

Jpn. Pat. Appln. Laid-Open Publication No. 2000-177921 describes a post-processing apparatus. In the apparatus described in the above-cited Patent Document, a pair of aligning members is arranged in a direction perpendicular to the conveyance direction of conveying sheets with a predetermined gap separating them and adapted to align sheets of a bundle. The alignment position for aligning sheets for bundles by means of the aligning members is shifted from bundle to bundle before the bundles of aligned sheets are delivered to a stack tray.

Thus, the post-processing apparatus of the above-cited Patent Document is designed to shift bundles of sheets from bundle to bundle instead of moving the stack tray in a direction perpendicular to the conveyance direction of conveying sheets of paper.

Jpn. Pat. Appln. Laid-Open Publication No. 2001-220055 described a sheet processing apparatus. In the apparatus described in this patent document, a pair of aligning members (side stoppers) is arranged in a direction perpendicular to the conveyance direction of conveying sheets with a predetermined gap separating them and adapted to align sheets of a bundle.

For binding a bundle of sheets at two positions by means of the sheet processing apparatus, the stapler is moved to the second stapling position after binding the bundle of sheets at the first stapling position. When the stapler is moved toward the second position, the side stoppers are moved in the direction opposite to the moving direction of the stapler so as to reduce the moving distance of the stapler.

However, with either of the apparatus described in Jpn. Pat. Appln. Laid-Open Publication No. 2000-177921 and Jpn. Pat. Appln. Laid-Open Publication No. 2001-220055, the stapled positions of some of the bundles of sheets can be aligned to heap up the bundles higher at the part of the binding positions than at the remaining part of the sheets. Additionally, a succeeding bundle of sheets that is stapled and delivered to the stack tray can force out one or more than one uppermost bundle of sheets already stored on the stack tray.

Therefore, the present invention provides a sheet post-processing apparatus that can reduce the risk of collapsing the heap of bundles of sheets of paper on a delivery tray by shifting the bound positions of an upper bundle of sheets from those of a lower bundle of sheets when delivering those bundles to the delivery tray and lay them one on the other after a stapling process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view of an embodiment of sheet post-processing apparatus according to the present invention;

FIG. 2 is a schematic plan view of the sheet post-processing apparatus of FIG. 1;

FIG. 3 is a schematic illustration of the configuration of a sheet post-processing apparatus according to the present invention;

FIG. 4 is a schematic perspective view of the stapler of a sheet post-processing apparatus according to the invention;

FIG. 5 is a schematic perspective view of the longitudinal alignment roller of a sheet post-processing apparatus according to the invention;

FIG. 6 is a schematic illustration of the paddle of a sheet post-processing apparatus according to the invention;

FIG. 7 is a schematic perspective view of the stand-by tray and the processing tray of a sheet post-processing apparatus according to the invention;

FIG. 8 is a schematic plan view of the stand-by tray and the processing tray of a sheet post-processing apparatus according to the invention;

FIG. 9 is a schematic perspective view of the transversal alignment plate and the conveyor belt of a sheet post-processing apparatus according to the invention;

FIG. 10 is a schematic illustration of the delivery operation of delivering sheets from a processing tray of a sheet post-processing apparatus according to the present invention;

FIG. 11 is a schematic illustration of the movement of the stand-by tray of a sheet post-processing apparatus according to the present invention;

FIG. 12 is a schematic illustration of the stapling operation of an ordinary sheet post-processing apparatus;

FIG. 13 is a schematic illustration of the basic operation of the stapler of a sheet post-processing apparatus according to the present invention;

FIG. 14A and FIG. 14B are schematic illustrations of the operation of the stapler of a sheet post-processing apparatus according to the present invention; and

FIG. 15A, FIG. 15B and FIG. 15C are schematic illustrations of the operation of delivering the sheets that have been stapled by a sheet post-processing apparatus according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Throughout this description, the embodiments and examples shown should be considered as exemplars, rather than limitations on the apparatus of the present invention.

Now, a preferred embodiment of the present invention will be described in detail by referring to the accompanying drawings. Throughout the drawings, the same parts are denoted by the same reference symbols and will not be described repeatedly.

FIG. 1 is a schematic perspective view of an embodiment of sheet post-processing apparatus according to the present invention. FIG. 2 is a schematic plan view of the apparatus of FIG. 1. FIG. 3 is a schematic illustration of the configuration of a sheet post-processing apparatus according to the present invention. FIGS. 4 through 11 schematically illustrate different parts of a sheet post-processing apparatus according to the present invention.

The configuration and the operation of each part shown in FIGS. 1 and 2 will be described below by referring to FIG. 4 and the subsequent drawings. Firstly, processing for a sheet by a sheet post-processing apparatus according to the invention will be described mainly by referring to FIG. 3.

As shown in FIG. 3, sheet P on which an image is formed by an image forming apparatus 5 such as a copying machine is discharged from a pair of delivery rollers 6 and conveyed to the sheet post-processing apparatus 7. The sheet post-processing apparatus 7 includes a stand-by tray 10, a processing tray 12, a stapler 14, a first delivery tray 16, a second delivery tray 18, a gate G and so on.

The sheet P discharged from the pair of delivery rollers 6 of the image forming apparatus 5 is received by an entrance roller pair 22 arranged near the sending-in entrance of the sheet post-processing apparatus 7. The entrance roller pair 22 includes an upper roller 22 a and a lower roller 22 b. The entrance roller pair 22 is driven by a motor 26 (FIG. 1).

A gate G is arranged at the downstream side of the entrance roller pair 22. The gate G sorts the sheets P received by the entrance rollers 22 into two paths (flows) The gate G shows a wedge-shaped cross section and the narrow end of the wedge is directed to the entrance rollers 22. The gate G is rotatably supported by a lateral wall section in the sheet post-processing apparatus 7. The gate G is adapted to selectively take a first position where the narrow end thereof is directed to the upper entrance roller 22 a or a second position where the narrow end thereof is directed to the lower entrance roller 22 b.

The first position is to be used to select the path for sheets P that require post-processing, whereas the second position is to be used to select the path for sheets P that does not require any post-processing.

When the gate G is at the first position, each sheet P is supplied to the first sheet feeding roller pair 24 and then from the first sheet feeding roller pair 24 to the stand-by tray 10. A paper path ceiling 36 (FIG. 1) is arranged between the entrance rollers 22 to the stand-by tray 10 to guide the sheet P to a first sheet feeding roller pair 24. The first sheet feeding roller pair 24 includes an upper roller 24 a and a lower roller 24 b.

The stand-by tray 10 receives a plurality of sheets P and lays them one on the other there until the ongoing processing at the processing tray 12 ends. The processing tray 12 aligns and supports the laid sheets P until the end of the ongoing stapling process at the stapler 14, which is a processing mechanism for post-processing.

As a predetermined number of sheets are stacked in the stand-by tray 10, the tray members 10 a, 10 b are opened by a motor 34 (FIG. 1) in respective directions indicated by arrows n and m in FIG. 7. Then, as a result, the sheets P drop into the processing tray 12 by their own weights and fed to the stapler 14.

As shown in FIG. 4, the stapler 14 is driven to slide in the directions indicated by arrows u by a staple drive section 49 so as to be positioned for a stapling process. While only a single stapler 14 is provided, FIG. 4 shows the stapler before it is driven to slide and after it has been driven.

The processing tray 12 has a pair of longitudinal alignment rollers 38 a, 38 b as shown in FIGS. 5 and 6.

The upper and lower longitudinal alignment rollers 38 a, 38 b are also used to pinch the stapledbundleof sheets T between them and take it out from the stapler 14 after the stapling process. The longitudinal alignment roller 38 a is driven by motor 40, whereas the longitudinal alignment roller 38 b is driven by motor 42.

For the sheets P that falls so as to be supplied to the processing tray 12, a rotatable paddle 44 is arranged at the position where the rear ends of the sheets P are placed. The paddle 44 is adapted to longitudinally align the uppermost sheet P of the sheets that are laid on the processing tray 12.

As shown in FIG. 6, the paddle 44 includes a receiving section 44 a for receiving the sheets P that enter the stand-by tray 10, a slapping section 44 b for slapping down the sheets P on the processing tray 12 and a feed section 44 c for aligning the sheets P on the processing tray 12 and is driven by a paddle motor 46 (FIG. 5). The paddle 44 is made of rubber and hence is elastic.

A stopper 45 is arranged at the end of the processing tray 12 located at the side of the stapler 14 so as to contact the rear ends of the sheets P and regulate the positions of the rear ends. A conveyor belt 50 is arranged substantially at the center of the processing tray 12. The conveyor belt 50 is adapted to convey the bundle of sheets T that has been subjected to a stapling process and taken out from the stapler 14 by the longitudinal alignment rollers 38 a, 38 b to either the first delivery tray 16 or the second delivery tray 18. A feed claw 50 a for hooking the rear end of the bundle of sheets T is fitted to the conveyor belt 50.

While the stand-by tray 10 can fall and supply sheets P to the processing tray 12, it can also be used to convey sheets P either toward the first delivery tray 16 or toward the second delivery tray 18. More specifically, when sheets P are conveyed toward the delivery tray 16 or toward the delivery tray 18, the rotary roller pair 28 for aligning sheets P is brought into contact with the sheets P laid on the stand-by tray 10. The rotary roller pair 28 is controlled for its vertical movement by a drive source 30 and driven to rotate by a motor 32 (FIG. 2).

As shown in FIG. 3, the stand-by tray 10 is so arranged as to show an angle of inclination θ1 so as to support sheets P in a condition where the front ends of the sheets P are found above the respective rear ends thereof. The first delivery tray 16 or the second delivery tray 18 is selected and driven to move up and down by a drive section 52.

Thus, when receiving sheets P, the first delivery tray 16 or the second delivery tray 18 that is selected is raised or lowered substantially to the height of the processing tray 12 to align the sheets P discharged onto it better. The first delivery tray 16 or the second delivery tray 18 that is selected is so arranged as to show an angle of inclination θ2 so as to support sheets P in a condition where the front ends of the sheets P are found above the respective rear ends thereof.

As shown in FIGS. 7 and 8, the stand-by tray 10 is provided with a pair of tray members 10 a, 10 b that are formed to project from the wall surface thereof and adapted to slide sideways to receive sheets P and support them at the corresponding opposite ends thereof. The tray members 10 a, 10 b are provided respectively with stoppers 10 c, 10 d for regulating the rear ends of the received sheets P.

The stand-by tray 10 is driven to slide by a motor 34 (FIG. 2). A pair of transversal alignment plates 47 a, 47 b is arranged between the stand-by tray 10 and the processing tray 12 as shown in FIG. 9 in order to prevent the sheets P fallen from the stand-by tray 10 and supplied to the processing tray 12 from being disordered in the transversal direction that is orthogonal to the sheet conveying direction and transversally align them. The transversal alignment plates 47 a, 47 b can be driven to slide in the directions indicated by arrow v so as to make the gap between them match the width of the sheets P. In other words, their alignment positions can be shifted.

When the gate G is at the second position as shown in FIG. 3, a sheet P that does not require post-processing is fed to the second sheet feeding roller pair 60 and then further to the third sheet feeding roller pair 61. Each of the second sheet feeding roller pair 60 and the third sheet feeding roller pair 61 includes an upper sheet feeding roller and a lower sheet feeding roller. The sheet P conveyed out from the third sheet feeding roller pair 61 is sent to the fixed tray 19 arranged on the top surface of the sheet post-processing apparatus 7.

The motors 26, 32, 34, 40, 42, 46 and 48 for driving the above-described various mechanisms and the drive sections 49 and 52 are driven and controlled by a control circuit (not shown).

Now, the operation of the sheet post-processing apparatus 7 will be described in terms of the flow of sheets. As a sheet P on which an image is formed by the image forming apparatus 5 is supplied form the delivery roller pair 6, the sheet post-processing apparatus 7 operates differently depending on (1) when the sheet P is not to be subjected to post-processing, (2) when the sheet P is to be subjected to post-processing and the post-processing of the preceding sheet P has not been completed and (3) when the sheet P is to be subjected to post-processing and the post-processing of the preceding sheet P has been completed.

Firstly, (1) when the sheet P is not to be subjected to post-processing, the narrowed part of the wedge-shaped gate G is positioned to substantially point the lower entrance roller 22 b. The sheet P delivered from the entrance roller pair 22 is supplied to the second sheet feeding roller pair 60 and then further to the third sheet feeding roller pair 61. The sheet P that is fed out from the third sheet feeding roller pair 61 is delivered to the fixed tray 19 on the top surface of the sheet post-processing apparatus.

Now, (2) when the sheet P is to be subjected to post-processing (a stapling process) and there is no preceding sheet P on the processing tray 12, the stand-by tray 10 is driven to slide in the direction of arrow m or arrow n to either of the positions indicated by dotted lines, where it drops the sheet P. The transversal alignment plates 47 a, 47 b are arranged so as to show a gap between them substantially equal to the width of the sheet P in order to transversally align the dropping sheet P. Then, as a result, the sheet P supplied by the sheet feeding roller pair 24 is directly dropped onto the processing tray 12 without being obstructed by the stand-by tray 10 on the way.

When the sheet P is dropped, the longitudinal alignment roller pair 38 a is retreated upwardly and the receiving section 44 a of the paddle 44 receives the sheet P at the rear end of the sheet P. The sheet P falls as its two lateral sides are guided by the transversal alignment plates 47 a, 47 b so as to be aligned in the transversal direction. Thereafter, the paddle 44 is driven to rotate in the sense as indicated by arrow o in FIG. 6 in order to drop the sheet P from its receiving section 44 a from the rear end of the sheet P and slaps it down onto the processing tray 12 by means of its slapping section 44 b.

Additionally, the paddle 44 sends out the sheet P in the direction of arrow q by means of its feed section 44 c until the rear end of the sheet P contacts the stopper 45 to complete the operation of longitudinally aligning the sheet P. Note that the longitudinal alignment roller 38 a may be driven to move up and down each time a sheet P is delivered to it in order to perform the operation of longitudinally aligning the sheet P on the processing tray 12.

Thus, the sheet P on which an image is formed is directly laid on the processing tray 12 from the sheet feeding roller pair 24 while it is sequentially aligned transversally and longitudinally. As the number of sheets P on the processing tray 12 gets to a predetermined level, the sheets P on the processing tray 12 is stapled by the stapler 14 at one or more than one desired positions to form a bundle of sheets T.

Thereafter, the bundle of sheets T is pinched by the longitudinal alignment roller 38 a that is driven to rotate in the sense of arrow r and the longitudinal alignment roller 38 b that is driven to rotate in the sense of arrow s in FIG. 6 and conveyed toward the first delivery tray 16.

As the rear end of the bundle of sheets T passes the longitudinal alignment rollers 38 a, 38 b, it is hooked by the feed claw 50 a of the conveyor belt 50 that is driven to rotate in the sense of arrow t in FIG. 5 and put on the first delivery tray 16. At this time, the first delivery tray 16 is driven to slide from the position indicated by a dotted line to the position indicated by a solid line in FIG. 3.

Since the first delivery tray 16 is arranged with an angle of inclination θ2 and hence the front ends of the sheets P are found above the respective rear ends thereof, the preceding sheet P delivered onto the first delivery tray 16 would not be pushed out if the rear end of the preceding sheet P touches the front end of the succeeding bundle of sheets T.

If the sheets of the preceding bundle of sheets T are misaligned by the succeeding sheet P, the bundle of sheets T falls by its own weight and hence is laid on the first delivery tray 16 with the rear ends of the sheets aligned with each other because of the angle of inclination θ2. In other words, the delivered sheets P are sequentially laid on the first delivery tray 16 in the proper order to complete the process of stapling the sheets P.

Thus, sheets are sequentially laid on the first delivery tray 16 in the above-described manner. As pointed out above, the first delivery tray 16 is made to show an angle of inclination of θ2 and hence the front ends of the sheets P on it are found above the respective rear ends thereof. Therefore, if a sheet P is curled to raise a middle part from the other parts thereof when delivered onto the first delivery tray 16 and its front end touches the preceding sheet P laid on the first delivery tray 16, the preceding sheet P would not be pushed out by the succeeding curled sheet P. In other words, the delivered sheets P are sequentially laid on the first delivery tray 16 in the proper order.

Finally, the instance (3) when the sheet P is to be subjected to post-processing and there are preceding sheets P remaining on the processing tray 12 because the stapling process that is being executed on them has been completed will be described below.

In this condition, the tray members 10 a, 10 b are driven to slide respectively from the positions indicated by dotted lines in the directions opposite to arrows m and n to get to the positions indicated by solid lines in FIG. 11 so that they can support sheets P. Additionally, the rotary roller pair 28 is retreated above the stand-by tray 10 so as not to obstruct the sheets P. Thus, the sheet P discharged from the image forming apparatus 5 and delivered by the sheet feeding roller pair 24 is temporarily laid on the stand-by tray 10 until the processing tray 12 becomes ready for receiving it.

The second and subsequent sheets P laid on the stand-by tray 10 fall onto the stand-by tray 10 and then are sent toward the stoppers 10 c, 10 d by the rotary roller pair 28 that is driven to rotate in the sense opposite to that of arrow f in FIG. 3 until the rear ends of the sheets P contact the stoppers 10 c, 10 d so as to be longitudinally aligned. Additionally, the stand-by tray 10 is arranged to show an angle of inclination of θ1 so that the front ends of the sheets P on the stand-by tray 10 are found above the respective rear ends thereof. Thus, the sheets P are made to contact the stoppers 10 c, 10 d by their own weights and hence longitudinally aligned.

In this way, the supplied sheets P are sequentially laid on the stand-by tray 10 in the proper order. If a preceding sheet P is pushed by a succeeding sheet P and slightly misaligned, the preceding sheet P falls to the position where its rear end contacts the stoppers 10 c, 10 d because of the angle of inclination θ1. Thus, the rear ends of the sheets P that are laid on the stand-by tray 10 are held to an aligned state.

Meanwhile, as the preceding sheets P on the processing tray 12 are discharged toward the first delivery tray 16 and the processing tray 12 is ready for receiving the succeeding sheets P, the tray members 10 a, 10 b of the stand-by tray 10 are driven to slide respectively in the direction of arrow m and the direction of arrow n from their positions indicated by solid lines to the positions indicated by dotted lines by way of the positions indicated by dotted chain lines in FIG. 11.

Then, as the tray members 10 a, 10 b get to the respective positions indicated by dotted chain lines in FIG. 11, the sheets P, e.g., two sheets P, that are on a stand-by status on the stand-by tray 10 are made to fall onto the processing tray 12 between the tray members 10 a, 10 b. At this time, the gap separating the transversal alignment plates 47 a, 47 b is made to be substantially equal to the width of the sheets P. Thus, the sheets P that are made to fall from the stand-by tray 10 are restricted at the lateral sides thereof and transversally aligned by the transversal alignment plates 47 a, 47 b.

The lower sheet P of the two sheets that are made to fall on the processing tray 12 is sent in the direction of arrow q by the longitudinal alignment roller 38 b that is driven to rotate in the sense opposite to that of arrow s in FIG. 6 so as to contact the stopper 45 at the rear end of thereof to complete the operation of aligning the sheet P in the longitudinal direction.

On the other hand, the upper sheet P of the two sheets that are made to fall on the processing tray 12 is sent in the direction of arrow q by the longitudinal alignment roller 38 a that is driven to rotate in the sense opposite to that of arrow r in FIG. 6 so as to contact the stopper 45 at the rear end thereof to complete the operation of aligning the sheet P in the longitudinal direction. Thereafter, the longitudinal alignment roller 38 a is retreated upwardly.

Then, the third and subsequent sheets P discharged from the image forming apparatus 5 are made to fall directly from between the tray members 10 a, 10 b to the processing tray 12 without held to a stand-by status on the stand-by tray 10. The third and subsequent sheets P are sequentially aligned on the sheets P that have been laid on the processing tray 12 by the paddle 44.

When the number of sheets P laid on the processing tray 12 gets to a predetermined level, a bundle of sheets T is formed by the stapler 14 by way of a stapling process. Thereafter, the bundle of sheets T is conveyed toward the first delivery tray 16 by the longitudinal alignment rollers 38 a, 38 b and hooked by the feed claw 50 a of the conveyor belt 50 at the rear end thereof so as to be put on the first delivery tray 16 as shown in FIG. 10 to complete the stapling process of the sheets P.

The overall operation of the sheet post-processing apparatus 7 is described above. Now, the configuration of the stapling process section that characterized the present invention will be described below.

Generally, sheets are stapled at a corner or at a plurality of (e.g., two) positions along an edge of the sheets. When sheets are stapled at a plurality of positions, bundles of sheets T are stacked on the delivery tray 16 and the staples are laid one on the other at staple positions ST to heap up as shown in FIG. 12. Then, a succeeding bundle of sheets T that is stapled and delivered to the delivery tray can force out one or more than one uppermost bundle of sheets T already delivered to the tray 16 to collapse the stack.

A sheet post-processing apparatus according to the invention is characterized in that it can prevent staples from being laid one on the other to form a heap. Then, it is possible to prevent the stack of bundles of sheets on the stack tray from collapsing.

FIG. 13 is a schematic illustration of the basic operation of the stapler 14 of a sheet post-processing apparatus according to the present invention. As shown in FIG. 4, the stapler 14 can be moved in the direction of u by the staple drive section 49. Therefore, when sheets P are bound at a corner, the stapler is moved to the position indicated by solid lines or dotted chain lines in FIG. 13 for a stapling process. On the other hand, when sheets P are bound at a plurality of (e.g., two) positions along an edge of sheets P, the stapler is moved to the positions indicated by dotted lines in FIG. 13 for a stapling process ST.

FIG. 14A and FIG. 14B are schematic illustrations of the operation of the stapler of a sheet post-processing apparatus according to the present invention when stapling sheets at two positions along an edge of sheets P. They show how the stapler 14 and the transversal alignment plates 47 a, 47 b are moved.

FIG. 14A illustrates that sheets P are aligned at the first alignment position by the transversal alignment plates 47 a, 47 b. In this condition, the sheets P are aligned transversally at the two lateral sides thereof in the direction along which the sheets P are conveyed. The transversal alignment plates 47 a, 47 b are located at respective positions indicated by b1 and b2.

Meanwhile, the stapler 14 is driven to slide along an edge of the sheets P by the staple drive section 49 in the direction of u and stops at the first staple position a1 for a stapling operation of the stapling process. Then, the stapler 14 is further driven to slide along the edge of the sheets P in the direction of arrow A1 and stops at the second staple position a2 for another stapling operation of the stapling process.

In this way, the stapler 14 is driven to move and stops at a plurality of positions for the stapling process. As the stapling process at the first and second staple positions ends, the sheets P are conveyed toward the delivery tray 16 (or 18) by the conveyor 50. Note that the gap L1 separating the first staple position a1 and the second staple position a2 is about 120 mm.

As the stapled bundle of sheets is discharged, the transversal alignment plates 47 a, 47 b are driven to move to a slight extent in a direction perpendicular to the moving direction of the sheets relative to the first alignment position as shown in FIG. 14B. Then, the next bundle of sheets P that are brought in are aligned in the transversal direction by the transversal alignment plates 47 a, 47 b that have been moved at the second alignment position. In this condition, the transversal alignment plates 47 a, 47 b are located at respective positions indicated by c1 and c2.

The stapler 14 is driven to move from the above-described second staple position a2 by a predetermined distance L2 in the direction in which the transversal alignment plates 47 a, 47 b have been moved (the direction indicated by arrow A2) and stops at the third staple position A3 for a stapling operation of the stapling process. Then, the stapler 14 is driven to move distance L1 in the opposite direction (the direction indicated by arrow A3) along an edge of the sheets P and stops at the fourth staple position a4 for another stapling operation of the stapling process.

In this way, the stapler 14 is driven to move in the opposite direction and stops at a plurality of positions for the stapling process. Since the sheets P are aligned at a position that corresponds to the distance by which the transversal alignment plates 47 a, 47 b are moved, they are stapled always at the right positions.

As the stapling operations at the third and fourth staple positions a3, a4 are completed, the sheets P are conveyed to the delivery tray 16 (or 18) by the conveyor belt 50 and the transversal alignment plates 47 a, 47 b return to the respective first alignment positions b1, b2 to align the next bundle of sheet P that are brought in while the staple 14 is driven to slide by the staple drive section 49 and returns to the first staple position a1. The above cycle is repeated for the stapling process to progress.

Thus, the bundles of sheets T discharged to the delivery tray 16 (or 18) are placed zigzag vertically but stapled at the same positions so long as the sheets have the same size. In other words, the staples gap of a preceding bungle of sheets T and those of the immediately succeeding bundle of sheets T that are discharged onto the delivery tray 16 are separated by a constant gap.

In the above-described operation of the stapler 14, the distance L1 from the first staple position a1 to the second staple position a2, which is equal to the distance from the third staple position a3 to the fourth staple position a4, is defined by the gap between the sheet-binding positions, which is about 120 mm.

The distance L2 from the second staple position a2 to the third staple position a3 may be appropriately selected so as to differentiate the staple positions ST at the first alignment position and the staple positions ST at the second alignment position. For instance, about 15 mm may be sufficient for the distance L2. Similarly, about 15 mm may be sufficient for the distance L2 from the fourth staple position a4 to the first staple position a1.

In this way, the stapler 14 is driven to move cyclically along the staple positions a1→a2→a3 and along the staple positions a3→a4→a1 according to the alignment positions of the alignment plates 47 a, 47 b. The moving distance of the stapler 14 can be reduced for each cycle if the moving distance L2 is defined to be not greater than L1/2 for instance. Then, the moving time can be reduced to realize a high speed stapling process.

FIGS. 15A through 15C schematically illustrate how the bundles of sheets T that have been stapled sequentially discharged to the delivery tray 16. FIG. 15A shows that bundles of sheets are brought in and laid zigzag and hence their stapling positions ST are arranged zigzag. FIG. 15B illustrates how staple positions ST are arranged in a zigzag manner for bundles T of sheets.

As shown in FIG. 15B, when sheets of a same size are stapled, the stapling positions are arranged zigzag without failling to reduce the height H1 of the stapled bundles of sheets. Therefore, if a large number of bundles of sheets T are laid on the delivery tray 16, the height of the heap of staples canbe reduced. Then, it is possible to minimize the problem that a succeeding bundle of sheets that is discharged to the delivery tray can force out one or more than one uppermost bundle of sheets T already delivered to the delivery tray.

FIG. 15C illustrates how bundles of sheets T are laid one on the other by a known sheet post-processing apparatus for the purpose of comparison. As shown, staples are placed at the same positions and laid one on the other to raise the height H2 of the stapled bundles of sheets (H2>H1). Then, the bundles of sheets on the delivery tray can easily collapse.

As described above, according to the present invention, it is possible to minimize the problem that a succeeding bundle of sheets that is stapled and discharged to the delivery tray 16 (or 18) can force out one or more than one uppermost bundle of sheets already discharged to the delivery tray to collapse the stack. Thus, a sheet post-processing apparatus according to the present invention does not reduce the efficiency of the corresponding image forming apparatus and hence is convenient to the users.

The present invention is by no means limited to the above-described embodiment, which may be modified and altered in various different ways without departing from the scope of the invention as defined in claims. For example, while the stapler 14 staples each bundle of sheets at two positions but it may alternatively staple at three positions.

Although exemplary embodiments of the present invention have been shown and described, it will be apparent to those having ordinary skill in the art that a number of changes, modifications or alternations to the invention as described herein may be made, non of which depart fro the spirit of the present invention. All such changes, modifications and alterations should therefore be seen as within the scope of the present invention. 

1. A sheet post-processing apparatus for aligning a plurality of sheets conveyed from an image forming apparatus and stapling and delivering them, said apparatus comprising: a processing tray capable of carrying a plurality of sheets conveyed from the image forming apparatus; alignment means including a pair of alignment plates for aligning the plurality of sheets carried by the processing tray at the two lateral sides running in the direction of conveyance of the plurality of sheets, the pair of alignment plates being movable between a first alignment position and a second alignment position displaced from the first alignment position by a predetermined distance in a direction perpendicular to the direction of conveyance; and staple means including a stapler movable along the leading edge of the plurality of sheets aligned by the alignment means in the direction of conveyance and adapted to move the stapler according to the change of alignment position in order to staple the conveyed sheets at a plurality of positions; the sheets stapled at the first alignment position and those stapled at the second alignment position being sequentially discharged and led to a delivery tray so as to shift the staple positions of the bundles of sheets stacked on the delivery tray in a zigzag manner.
 2. The apparatus according to claim 1, wherein the staple positions and the gap separating the staples are held same among the bundles of sheets when the bundles of sheets stapled by the staple means have the same size.
 3. The apparatus according to claim 2, wherein, when the gap separating the staples of each bundle of sheets stapled by the staple means is L1, the positional shift L2 of an upper bundle of sheets stacked on the delivery tray from a lower bundle of sheets stacked on the delivery tray is defined by L2<L1/2.
 4. A sheet post-processing apparatus for aligning a plurality of sheets conveyed from an image forming apparatus and stapling and delivering them, said apparatus comprising: a processing tray capable of carrying a plurality of sheets conveyed from the image forming apparatus; alignment means including a pair of alignment plates for aligning the plurality of sheets carried by the processing tray at the two lateral sides running in the direction of conveyance of the plurality of sheets, the pair of alignment plates being capable of reciprocating between a first alignment position and a second alignment position displaced from the first alignment position by a predetermined distance in a direction perpendicular to the direction of conveyance; a stapler movable along the leading edge of the plurality of sheets aligned by the alignment means in the conveyance direction; a control section for controlling the staple so as to cyclically move it along the staple positions according to the change of alignment position of the alignment means, the control section moving the stapler in the first direction when the alignment means is at the first alignment position and causing it to staple the sheets at a plurality of positions along the moving route, the control section moving the stapler in the direction opposite to the first direction when the alignment means is at the second alignment position and causing it to staple the sheets at a plurality of positions along the moving route; and a delivery section for leading the sheets stapled at the first alignment position and those stapled at the second alignment position to a delivery tray; the staple positions of the bundles of sheets stacked on the delivery tray being shifted in a zigzag manner.
 5. The apparatus according to claim 4, wherein the control means controls the position of the staple according to the alignment position in such a way that it moves the stapler in the first direction at the first alignment position and operates the stapler to staple the sheets at a first position and at a second position located along the moving direction but it moves the stapler to a third position separated from the second position by a predetermined distance in the first direction when the alignment means is driven to move to the second alignment position and it moves the stapler in the direction opposite to the first direction at the second alignment position and operates the stapler to staple the sheets at a third position and at fourth position located along the moving direction, while it moves the stapler to the first position separated from the fourth position by a predetermined distance when the alignment means is driven to return to the first alignment position.
 6. The apparatus according to claim 4, wherein when the gap separating the staples of each bundle of sheets stapled by the staple means at the first and second alignment positions is L1, the predetermined distance L2 by which the stapler is moved according to the move of alignment position of the alignment means is defined by L2<L1/2.
 7. The apparatus according to claim 4, wherein the alignment means includes a second alignment means for aligning the plurality of sheets at the leading edge thereof in the direction of conveyance in addition to the pair of alignment plates for aligning the plurality of sheets carried by the processing tray from the two lateral sides running in the direction of conveyance of the plurality of sheets.
 8. The apparatus according to claim 4, wherein the second alignment means is arranged at the stapler and includes a stopper for receiving the leading edge of the sheets in the direction of conveyance. 